Pronounced field emission from vertically aligned carbon nanotube blocks and bundles

Abstract

Field emission from aligned carbon nanotube (CNT) blocks and bundles grown by two different chemical vapor deposition (CVD) methods were investigated. A single CNT block consists of about 1600 double-walled nanotubes per μm2 of area with an intertube distance in the range 15–20 nm, while a bundle consists of a few multiwalled CNTs, which contact each other in the tip region. Hydrogen/water assisted catalytic CVD on a silicon wafer resulted in ultrahigh aspect ratios for the CNT blocks with extremely high field enhancement coefficients >11 000, a turn-on field of 0.36 V/μm, and good field emission stability. The water and hydrogen concentration ratio were found to be a critical parameter for establishing high quality, ultralong (up to 2 mm) CNT growth. The second CVD growth method used was a noncatalytic template assisted technique for bundle generation. Special self-organized anodic alumina films controlled the synthesis of highly aligned carbon nanotube arrays with the desired tube geometry and adjustable intertube distances. The CNT bundle generation was adjusted by the CNT tube diameter, wall thickness, and length-controlled growth. Subsequent etching of the Al2O3 template resulted in substrateless flexible CNT bundles permitting simple gate electrode integration process. Field emission tests showed that the field enhancement factor, turn-on field, and field emission current density of the CNT cathode arrays could be optimized by adjusting the block or bundle distance. The authors obtained, for the block emitter, a smaller turn-on field and better stability. The block emitter with the better performance was additionally coated with CVD grown ZnO nanoparticles to obtain further improvement. The ZnO nanoparticle diameter was in the range of 15–20 nm. First results showed partial clustering of the ZnO particles suggesting the need for further optimization of the deposition.